"Reading Accounting Information

Note that there's currently no systemd API to retrieve accounting information from cgroups. For now, if you need to retrieve this information use /proc/$PID/cgroup to determine the cgroup path for your process in the cpuacct controller (or whichever controller matters to you), and then read the attributes directly from the cgroup tree."

That's a shame, I didn't realise it didn't exist yet :/

(from http://www.freedesktop.org/wiki/Software/systemd/ControlGroupInterface/)